1. Field of the Invention
This invention relates to temperature control of sources of electromagnetic radiation in illumination and projection systems.
2. Description of the Related Art
The objective of systems that collect, condense, and couple electromagnetic radiation into a target such as a standard waveguide, e.g. a single fiber or fiber bundle, or output electromagnetic radiation to the homogenizer of a projector, is to maximize the brightness of the electromagnetic radiation at the target. There are several common systems for collecting and condensing light from a lamp for such illumination and projection applications.
U.S. Pat. No. 4,757,431 (xe2x80x9cthe ""431 patentxe2x80x9d), the disclosure of which is incorporated by reference, describes a light condensing and collecting system employing an off-axis spherical concave reflector to enhance the flux illuminating a small target and the amount of collectable flux density reaching the small target. Another light condensing and collecting system is provided by U.S. Pat. No. 5,414,600 (xe2x80x9cthe ""600 patentxe2x80x9d), the disclosure of which is incorporated by reference, describes the use of an ellipsoid concave reflector. Similarly, U.S. Pat. No. 5,430,634 (xe2x80x9cthe ""634 patentxe2x80x9d), the disclosure of which is incorporated by reference, describes the use of a toroid concave reflector.
U.S. patent application Ser. No. 09/604,921, the disclosure of which is incorporated by reference, provides a dual-paraboloid reflector system. This optical collection and condensing system, as illustrated in FIG. 1, uses two generally symmetric paraboloid reflectors 10, 11 that are positioned so that light reflected from the first reflector 10 is received in a corresponding section of the second reflector 11. In particular, light emitted from a light source 12, such as an arc lamp, is collected by the first parabolic reflector 10 and collimated along the optical axis toward the second reflector 11. The second reflector 11 receives the collimated beam of light and focuses this light at the target 13 positioned at the focal point.
The optical system of FIG. 1 may employ a retro-reflector 14 in conjunction with the first paraboloid reflector 10 to capture radiation emitted by the light source 12 in a direction away from the first paraboloid reflector 10 and reflect the captured radiation back through the light source 12. In particular, the retro-reflector 14 has a generally spherical shape with a focus located substantially near the light source 12 (i.e., at the focal point of the first paraboloid reflector) toward the first paraboloid reflector to thereby increase the intensity of the collimated rays reflected therefrom.
U.S. application Ser. No. 09/669,841, the disclosure of which is incorporated by reference, describes a dual ellipsoidal reflector system. This optical collection and condensing system, as illustrated in FIG. 2, uses two generally symmetric ellipsoid reflectors 20, 21 that are positioned so that light reflected from the first reflector 20 is received in a corresponding section of the second reflector 21. In particular, light emitted from the light source 22 is collected by the first elliptical reflector 20 and focused at the optical axis 25 toward the second reflector 21. The second reflector 21 receives the diverged beam of light and focuses this light at the target 23 positioned at the focal point.
The systems described above are required to be efficient and have relatively long useful lives. Arc lamps, e.g. metal halide lamps, are often used in the above-mentioned systems as sources of light. Such arc lamps often have quartz envelopes. Quartz tends to react with metal halides at temperatures above about 1100xc2x0 C. Thus, if the lamp temperature is allowed to exceed 1100xc2x0 C., the metal halide will react with the quartz. Such a reaction changes the chemical composition of the metal halide within the quartz envelope, reducing the efficiency of the metal halide as a light source. Furthermore, products of the reaction will be deposited on the quartz envelope, blocking some of the light.
On the other hand, if the temperature is allowed to get too low, cold spots will occur around the arc. Proper evaporation of the metal halide does not take place if the temperature is too low, as within such cold spots. This will also reduce the efficiency of the discharge. There thus exists a narrow range of temperatures within which a metal halide lamp operates most efficiently, commensurate with long life.
While the above embodiments describe the application of the temperature control system to a dual-paraboloid illumination system, the same can be applied to a conventional on-axis system to prolong the life of the lamp. The temperature control system could also be applied to, e.g. an on-axis ellipsoid reflector system, an on-axis parabolic reflector system, an off-axis concave reflector system. In those systems, the temperature control system will also control the heating/cooling systems such that the temperature of the lamp is maintained within the manufacturer""s specifications.
Therefore, there remains a need to provide a method of controlling the temperature of a metal halide lamp in a collecting and condensing system within a narrow range of temperatures.
A temperature control system for a source of electromagnetic radiation, such as an arc lamp, in a collecting and condensing system including a first reflector having a first focal point and a first optical axis and a second reflector having a second focal point and a second optical axis. The first and second reflectors may be placed substantially symmetrically to each other such that their optical axes are substantially collinear. The source may be located proximate to the first focal point of the first reflector to produce rays of radiation that reflect from the first reflector towards the second reflector and substantially converge at the second focal point. A sensor, such as a voltage or a temperature sensor, may be disposed proximate to the source, and produces an output which may be substantially proportional to an attribute of the source. A comparator compares the output to a predetermined value and produces a difference between the output and the predetermined value. A cooling device such as a fan placed proximate to the source has a cooling transmission such as an air flow to cool the source. The cooling transmission such as the air flow may be substantially proportional to the difference between the output and the predetermined value if the output is greater than the predetermined value. In the alternative, the cooling transmission such as the air flow may be substantially constant if the output is greater than the predetermined value. In either case, if the output is less than the predetermined value, the cooling transmission such as the air flow may be substantially zero.
The temperature control system may also include a heater placed proximate to the source which produces a heat flux. In this case the comparator compares the output to a second predetermined value, and produces a second difference between the output and the second predetermined value. The heat flux may be substantially constant or, in an alternative embodiment, may be substantially proportional to the second difference if the output is less than the second predetermined value. Otherwise, if the output is greater than the second predetermined value the heat flux may be substantially zero.
The above and other features and advantages of the present invention will be further understood from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.